Portable electronic devices, including those for mobile communications, microsensors, micro-electromechanical systems (MEMS), and microfluidic devices will benefit from advances in energy storage. The availability of power sources with higher energy density and lower cost will enable a wider range of usage and functionality. One possible higher energy density source is the fuel cell.
For electronic devices with small power requirements, microfabricated power sources, including fuel cells, are being investigated. Issues to consider include reducing size and weight, improving signal integrity with fewer interconnects, increasing processing efficiency, and lowering cost.
Some fuels of interest in micro-fuel cells for devices include hydrogen, methanol, and other hydrocarbons (e.g., ethylene glycol or formic acid). Hydrogen fuel cells and direct methanol fuel cells (DMFCs) operate at relatively low temperature (e.g., ambient to 120° C.). They employ a solid proton exchange membrane (PEM) to transport the protons from the anode to the cathode. Hydrogen can be stored as a pressured gas or in a metal hydride form. It requires humidification for high membrane conductivity.
A methanol-water mixture can be oxidized at the anode in either liquid or vapor form. Methanol is an attractive fuel because it can be stored as a liquid, is inexpensive, and has a high specific energy. Compared with other fuel cell systems, the liquid-feed DMFC is relatively simple and could be easily miniaturized since it does not need a fuel reformer, complicated humidification, or thermal management system. Furthermore, methanol has a high energy density in comparison with lithium polymer and lithium ion polymer batteries.
Proton exchange membranes can be used in low-temperature fuel cells that operate with either hydrogen or methanol. The solid membrane in conventional fuel cells is usually a perfluorinated polymer with sidechains terminating in sulfonic acid moieties, such as Nafion™. Membranes in PEM fuel cells generally contain water to keep the conductivity high. Methanol crossover causes a mixed potential and poisoning of the oxygen reduction reaction, leading to decreased performance. Therefore, there is a need in the industry to overcome at least some of the aforementioned inadequacies and deficiencies.